Модель веб-приложений Comet: история и эволюция

Web application model
Comet is a web application model in which a long held HTTPS request allows a web server to push data to a browser, without the browser explicitly requesting it. Comet is an umbrella term, encompassing multiple techniques for achieving this interaction. All these methods rely on features included by default in browsers, such as JavaScript, rather than on non default plugins. The Comet approach differs from the original model of the web, in which a browser requests a complete web page at a time. Reverse Ajax, Two way web, and
HTTP server push
among others. The term Comet is not an acronym, but was coined by Alex Russell in his 2006 blog post. In recent years, the standardisation and widespread support of WebSocket and Server sent events has rendered the Comet model obsolete.

The ability to embed Java applets into browsers (starting with Netscape Navigator 2.0 in March 1996) made two way sustained communications possible, using a raw TCP socket to communicate between the browser and the server. This socket can remain open as long as the browser is at the document hosting the applet. Event notifications can be sent in any format text or binary and decoded by the applet.

The very first application using browser to browser communications was Tango Interactive, implemented in 1996–98 at the Northeast Parallel Architectures Center (NPAC) at Syracuse University using DARPA funding. TANGO architecture has been patented by Syracuse University. TANGO framework has been extensively used as a distance education tool. The framework has been commercialized by CollabWorx and used in a dozen or so Command&Control and Training applications in the United States Department of Defense.

The first set of Comet implementations dates back to 2000,Unreliable source: date=November 2017 with the Pushlets, Lightstreamer, and KnowNow projects. Pushlets, a framework created by Just van den Broecke, was one of the first open source implementations. Pushlets were based on server side Java servlets, and a client side JavaScript library. Bang Networks a Silicon Valley start up backed by Netscape co founder Marc Andreessen had a lavishly financed attempt to create a real time push standard for the entire web. In April 2001, Chip Morningstar began developing a Java based (J2SE) web server which used two HTTP sockets to keep open two communications channels between the custom HTTP server he designed and a client designed by Douglas Crockford; a functioning demo system existed as of June 2001. The server and client used a messaging format that the founders of State Software, Inc. assented to coin as JSON following Crockford's suggestion. The entire system, the client libraries, the messaging format known as JSON and the server, became the State Application Framework, parts of which were sold and used by Sun Microsystems, Amazon. com, EDS and Volkswagen. In March 2006, software engineer Alex Russell coined the term Comet in a post on his personal blog. The new term was a play on Ajax (Ajax and Comet both being common household cleaners in the USA). In 2006, some applications exposed those techniques to a wider audience: Meebo’s multi protocol web based chat application enabled users to connect to AOL, Yahoo, and Microsoft chat platforms through the browser; Google added web based chat to Gmail; JotSpot, a startup since acquired by Google, built Comet based real time collaborative document editing. New Comet variants were created, such as the Java based ICEfaces JSF framework (although they prefer the term "Ajax Push"

Comet applications attempt to eliminate the limitations of the page by page web model and traditional polling by offering two way sustained interaction, using a persistent or long lasting HTTP connection between the server and the client. Since browsers and proxies are not designed with server events in mind, several techniques to achieve this have been developed, each with different benefits and drawbacks. The biggest hurdle is the HTTP 1.1 specification, which states "this specification encourages clients to be conservative when opening multiple connections". Therefore, holding one connection open for real time events has a negative impact on browser usability: the browser may be blocked from sending a new request while waiting for the results of a previous request, e. g., a series of images. This can be worked around by creating a distinct hostname for real time information, which is an alias for the same physical server. This strategy is an application of domain sharding. Specific methods of implementing Comet fall into two major categories: streaming and long polling.

A basic technique for dynamic web application is to use a hidden iframe HTML element (an inline frame, which allows a website to embed one HTML document inside another). This invisible iframe is sent as a chunked block, which implicitly declares it as infinitely long (sometimes called "forever frame"). As events occur, the iframe is gradually filled with script tags, containing JavaScript to be executed in the browser. Because browsers render HTML pages incrementally, each script tag is executed as it is received. Some browsers require a specific minimum document size before parsing and execution is started, which can be obtained by initially sending 1–2 kB of padding spaces. One benefit of the iframes method is that it works in every common browser. Two downsides of this technique are the lack of a reliable error handling method, and the impossibility of tracking the state of the request calling process. That is, if the main web page is served from one SLD, and the Comet server is located at another SLD (which does not have cross origin resource sharing enabled), Comet events cannot be used to modify the HTML and DOM of the main page, using those transports. This problem can be sidestepped by creating a proxy server in front of one or both sources, making them appear to originate from the same domain. However, this is often undesirable for complexity or performance reasons. Unlike iframes or XMLHttpRequest objects, script tags can be pointed at any URI, and JavaScript code in the response will be executed in the current HTML document. This creates a potential security risk for both servers involved, though the risk to the data provider (in our case, the Comet server) can be avoided using JSONP. A long polling Comet transport can be created by dynamically creating script elements, and setting their source to the location of the Comet server, which then sends back JavaScript (or JSONP) with some event as its payload. Each time the script request is completed, the browser opens a new one, just as in the XHR long polling case. This method has the advantage of being cross browser while still allowing cross domain implementations. which defines a new JavaScript interface EventSource and a new MIME type text/event stream. All major browsers except Microsoft Internet Explorer include this technology. The HTML 5 WebSocket API working draft specifies a method for creating a persistent connection with a server and receiving messages via an onmessage callback. The Bayeux protocol by the Dojo Foundation. It leaves browser specific transports in place, and defines a higher level protocol for communication between browser and server, with the aim of allowing re use of client side JavaScript code with multiple Comet servers, and allowing the same Comet server to communicate with multiple client side JavaScript implementations. Bayeux is based on a publish/subscribe model, so servers supporting Bayeux have publish/subscribe built in. The BOSH protocol by the XMPP standards foundation. It emulates a bidirectional stream between the browser and server by using two synchronous HTTP connections. The JSONRequest object, proposed by Douglas Crockford, would be an alternative to the XHR object. Use of plugins, such as Java applets or the proprietary Adobe Flash (using RTMP protocol for data streaming to Flash applications). These have the advantage of working identically across all browsers with the appropriate plugin installed and need not rely on HTTP connections, but the disadvantage of requiring the plugin to be installed
Google announced a new Channel API for Google App Engine, implementing a Comet like API with the help of a client JavaScript library on the browser. This API has been deprecated.